Large‐scale melt‐depletion in granulite terranes: an example from the Archean Ashuanipi Subprovince of Quebec

Guernina, Souad; Sawyer, E. W.

doi:10.1046/j.1525-1314.2003.00436.x

Cited by 153 publications

(97 citation statements)

References 89 publications

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“…Extrapolation to the full extent of the Ashuanipi Subprovince suggests that approximately 6.5 × 10 5 km 3 of melt could have been extracted from granulite facies middle-to-lower crust; this would represent an equivalent 'layer' of granite some 7 km thick (Guernina & Sawyer 2003). …”

Section: (B) the Length Scales And Time Scales Of Melt Flow And The mentioning

confidence: 99%

The spatial and temporal patterning of the deep crust and implications for the process of melt extraction

Brown

2010

Phil. Trans. R. Soc. A.

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Volumetrically significant melt production requires crustal temperatures above approximately 800• C. At the grain scale, the former presence of melt may be inferred based on various microstructures, particularly pseudomorphs of melt pores and grainboundary melt films. In residual migmatites and granulites, evidence of melt-extraction pathways at outcrop scale is recorded by crystallized products of melt (leucosome) and residual material from which melt has drained (melanosome). These features form networks or arrays that potentially demonstrate the temporal and spatial relations between deformation and melting. As melt volume increases at sites of initial melting, the feedback between deformation and melting creates a dynamic rheological environment owing to localization and strain-rate weakening. With increasing temperature, melt volume increases to the melt connectivity transition, in the range of 2-7 vol% melt, at which point melt may escape in the first of several melt-loss events, where each event represents a batch of melt that left the source and ascended higher in the crust. Each contributing process has characteristic length and time scales, and it is the nonlinear interactions and feedback relations among them that give rise to the dissipative structures and episodicity of melt-extraction events that are recorded as variations in the spatial and temporal patterning of the crust. Focused melt flow occurs by dilatant shear failure of low-melt fraction rocks creating melt-flow networks that allow accumulation and storage of melt, and form the link for melt flow from grain boundaries to veins allowing drainage to crustal-scale ascent conduits. Preliminary indications suggest that anatectic systems are strongly self-organized from the bottom up, becoming more ordered by decreasing the number and increasing the width of ascent conduits from the anatectic zone through the overlying subsolidus crust to the ductile-to-brittle transition zone, where the melt accumulates in plutons.

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Section: (B) the Length Scales And Time Scales Of Melt Flow And The mentioning

confidence: 99%

The spatial and temporal patterning of the deep crust and implications for the process of melt extraction

Brown

2010

Phil. Trans. R. Soc. A.

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“…(ii) What controls the level at which this magma accumulates in the crust? (iii) Is there an optimal set of conditions that produces giant granitoid complexes such as occur in the Ashuanipi sub province, Quebec (Gurnina & Sawyer 2003) and the Lachlan Fold Belt of Eastern Australia (Vernon & Clarke 2008)? (iv) What sets the upper limit to the production and size of such systems?…”

Section: Entropy Production Rate In Granitoid Systemsmentioning

confidence: 99%

The mechanics of granitoid systems and maximum entropy production rates

Hobbs

Ord

2010

Phil. Trans. R. Soc. A.

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A model for the formation of granitoid systems is developed involving melt production spatially below a rising isotherm that defines melt initiation. Production of the melt volumes necessary to form granitoid complexes within 10 4 -10 7 years demands control of the isotherm velocity by melt advection. This velocity is one control on the melt flux generated spatially just above the melt isotherm, which is the control valve for the behaviour of the complete granitoid system. Melt transport occurs in conduits initiated as sheets or tubes comprising melt inclusions arising from GursonTvergaard constitutive behaviour. Such conduits appear as leucosomes parallel to lineations and foliations, and ductile and brittle dykes. The melt flux generated at the melt isotherm controls the position of the melt solidus isotherm and hence the physical height of the Transport/Emplacement Zone. A conduit width-selection process, driven by changes in melt viscosity and constitutive behaviour, operates within the Transport Zone to progressively increase the width of apertures upwards. Melt can also be driven horizontally by gradients in topography; these horizontal fluxes can be similar in magnitude to vertical fluxes. Fluxes induced by deformation can compete with both buoyancy and topographic-driven flow over all length scales and results locally in transient 'ponds' of melt. Pluton emplacement is controlled by the transition in constitutive behaviour of the melt/magma from elastic-viscous at high temperatures to elastic-plastic-viscous approaching the melt solidus enabling finite thickness plutons to develop. The system involves coupled feedback processes that grow at the expense of heat supplied to the system and compete with melt advection. The result is that limits are placed on the size and time scale of the system. Optimal characteristics of the system coincide with a state of maximum entropy production rate.

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“…Finally, a general model is proposed for the extraction of melt from partially melted middle and lower crust. This contribution focuses on the residual rocks and is part of a paper published in Journal of Metamorphic Geology (Guernina & Sawyer 2003).…”

Section: Methodology and Thesis Organizationmentioning

confidence: 99%

Formation and evolution of granite magmas from migmatites :

Guernina¹

2007

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Many magmas in the continental crust are believed to have formed by partial melting in the lower to middle crust, and the transfer of that melt to the upper crust. However, it is poorly known how the compositions of these magmas evolve. In general, the principal processes involved are redistribution and contamination of the melt fraction with some of the host-rock it passes through and once magma ascends out of the anatectic zone and traverses cooler crust and begins to crystallize. The nature of the protolithrnigmatite-granite transition and the process that creates a granitic magma during anatexis, as well as the processes that control the magma composition outside the source region are of a particular interest as they enable us to constrain the morphological, mineralogical and geochemical variations of anatectic melts produced during anatexis of greywacke metasedimentary rocks.The Ashuanipi subprovince and contiguous metasedimentary subprovinces in the Superior province contains reworked Archaean crust that displays a continuous section from mid-crustal anatectic rocks to shallow level granite plutons. In other words, the superior province represents a crustal section which is continuous from partially melted source to upper crustal magma sink and the Ashaunipi portion of it is an example of a source region for granulitic magma. There, the transition from palaesome to granite was an open system process through intermediate stages of metatexite and diatexite.Field, pétrographie and geochemical methods have been used to estimate how much granitic melt was formed and extracted from granulite facies terrane, and to determine what the grain and outcrop-scale melt-flow paths were during the melt segregation process. More than 85 % of the metasediments in the Ashuanipi subprovince are of metagreywacke composition that was metamorphosed at mid-crustal conditions (820-900 °C and 6-7 kbar). Decrease in modal biotite and quartz as orthopyroxene and plagioclase contents increase, together with preserved former melt microstructures indicate that anatexis was by the biotite dehydration reaction: biotite + quartz + plagioclase = melt + orthopyroxene + oxides. Using melt/orthopyroxene ratios for this reaction derived from experimental studies, the modal orthopyroxene contents present in the Ashuanipi rocks indicate that the metagreywacke rocks underwent an average of 31 vol% partial melting.The metagreywackes are enriched in MgO, CaO and FeO and depleted in SÍO2, K2O, Rb, Cs and U, have lower Rb/SR, higher Rb/Cs and Th/U ratios and positive Eu anomalies compared to their likely protolith. These compositions are modelled by the extraction of between 20 and 40 wt % granitic melt from typical Achaean low-grade metagreywackes.The distribution of relict melt at thin section and outcrop scales indicates that in layers without leucosomes melt extraction occurred by a pervasive grain boundary (porous) flow from the site of melting, across the layers and into bedding planes between adjacent layers. In other rocks, pervasive g...

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Large‐scale melt‐depletion in granulite terranes: an example from the Archean Ashuanipi Subprovince of Quebec

Cited by 153 publications

References 89 publications

The spatial and temporal patterning of the deep crust and implications for the process of melt extraction

The spatial and temporal patterning of the deep crust and implications for the process of melt extraction

The mechanics of granitoid systems and maximum entropy production rates

Formation and evolution of granite magmas from migmatites :

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